.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
.\" %%%LICENSE_START(VERBATIM)
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.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
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.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
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.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
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.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified Sat Jul 24 18:55:27 1993 by Rik Faith (faith@cs.unc.edu)
.TH MEMCMP 3  2021-03-22 "" "Linux Programmer's Manual"
.SH NAME
memcmp \- compare memory areas
.SH SYNOPSIS
.nf
.B #include <string.h>
.PP
.BI "int memcmp(const void *" s1 ", const void *" s2 ", size_t " n );
.fi
.SH DESCRIPTION
The
.BR memcmp ()
function compares the first \fIn\fP bytes (each interpreted as
.IR "unsigned char" )
of the memory areas \fIs1\fP and \fIs2\fP.
.SH RETURN VALUE
The
.BR memcmp ()
function returns an integer less than, equal to, or
greater than zero if the first \fIn\fP bytes of \fIs1\fP is found,
respectively, to be less than, to match, or be greater than the first
\fIn\fP bytes of \fIs2\fP.
.PP
For a nonzero return value, the sign is determined by the sign of
the difference between the first pair of bytes (interpreted as
.IR "unsigned char" )
that differ in
.I s1
and
.IR s2 .
.PP
If
.I n
is zero, the return value is zero.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR memcmp ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
.SH NOTES
Do not use
.BR memcmp ()
to compare security critical data, such as cryptographic secrets,
because the required CPU time depends on the number of equal bytes.
Instead, a function that performs comparisons in constant time is required.
Some operating systems provide such a function (e.g., NetBSD's
.BR consttime_memequal ()),
but no such function is specified in POSIX.
On Linux, it may be necessary to implement such a function oneself.
.SH SEE ALSO
.BR bcmp (3),
.BR bstring (3),
.BR strcasecmp (3),
.BR strcmp (3),
.BR strcoll (3),
.BR strncasecmp (3),
.BR strncmp (3),
.BR wmemcmp (3)
.SH COLOPHON
This page is part of release 5.13 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.
